Symmetric Cryptography

In this guided exercise, you will apply what you have learned to create and share a secret key with a second user. You will then assume the role of the second user and use the secret key to send an encrypted message to the first user. Finally, you will re-assume the role of the first user and decrypt the second user’s message. For the purposes of this exercise, the first user is named cybrary and the second user is named john.

1. On the dock at the bottom of the screen, click the Terminal Emulator icon to open a Terminal window.
   
2. At the cybrary@localhost prompt, type openssl genpkey -algorithm rsa -out keyforjohn.key and press Enter to create a new secret key that you will share only with John.
   

   

   Note: The algorithm used in the operation above, RSA, is actually an asymmetric cipher. However, when used in this manner - to generate a single private key, without its public key companion - the result is functionally equivalent to a symmetric cipher.
3. At the cybrary@localhost prompt, type cat keyforjohn.key and press Enter to display the secret key.
   

   

4. At the cybrary@localhost prompt, type base64 keyforjohn.key > keyforjohn.send and press Enter to encode the new key to base64.
   
   Encoding the key will make it easier to send via tools like mail.
   
5. At the cybrary@localhost prompt, type cat keyforjohn.send | mail -s "For John" john and press Enter to mail the shared key to the user john.
   
6. At the cybrary@localhost prompt, type su - john and press Enter to become the local user john.
   
   When prompted for the password, type password and press Enter.
   

   

   Notice that the prompt changes, indicating you are now the john user.
   
7. At the john@localhost prompt, type mail and press Enter to read John's local system mail.
   

   

   You should see a new message from Cybrary.
   
8. At the ? prompt, type 1 and press Enter to open the message.
   

   

   If you see the More prompt, press the space bar to see the entire message.
9. Highlight the message body (only the message body), then right-click and select Copy from the context menu.
   

   

10. At the ? prompt, type exit and press Enter to close the mail application.
    
11. At the john@localhost prompt, type echo " and paste the copied message contents, then continue typing " > keyforjohn.base64 and press Enter.
    

    

    This will look strange on screen, but the paste operation should work fine.
    
12. At the john@localhost prompt, type base64 -d keyforjohn.base64 > keyfromcybrary.key and press Enter to decode the shared key.
    
13. At the john@localhost prompt, type cat keyfromcybrary.key and press Enter. The key should look the same as the first key did in Step 3.
    
    Now that we have a shared key from Cybrary, we can send Cybrary encrypted messages!
    
14. At the john@localhost prompt, type echo "Be sure to drink your ovaltine!" > message.txt and press Enter to create the message.txt file.
    
15. At the john@localhost prompt, type openssl pkeyutl -encrypt -inkey keyfromcybrary.key -in message.txt -out message.enc and press Enter to encrypt the text message using the shared key from Cybrary.
    
16. At the john@localhost prompt, type cat message.enc and press Enter to display the encrypted message.
    

    

    It should appear as though the file contains gibberish. You cannot simply cut and paste the encrypted file contents as you would from a text file. We must encode this encrypted file to base64 before sending it to Cybrary using mail.
    
17. At the john@localhost prompt, type base64 message.enc > message.send and Press Enter to encode the encrypted message.
    
18. At the john@localhost prompt, type cat message.send | mail -s "Secret Message from John" cybrary and press Enter to mail the encoded message to Cybrary.
    
    Note: Encoding and encrypting are not the same thing. Encoding allows us to send things using mail that would not be possible otherwise.
    
19. At the john@localhost prompt, type exit and press Enter to become the cybrary user.
    
20. At the cybrary@localhost prompt, as the cybrary user, type mail and press Enter to open the mail application.
    
    You will see a new message from user john.
    
21. At the ? prompt, type 2 and press Enter to read the message from John.
    
22. Highlight the message body (only the message body), then right-click and select Copy from the context menu.
    
23. At the ? prompt, type exit and press Enter to quit the mail application.
    
24. At the cybrary@localhost prompt, type echo " and paste the copied message contents, then continue typing " > message.base64 and press Enter.
    
25. At the cybrary@localhost prompt, type base64 -d message.base64 > message.enc and press Enter to decode the encrypted message.
    
    You now have a decoded (but not decrypted) copy of the encrypted message.
    
26. At the cybrary@localhost prompt, type openssl pkeyutl -decrypt -inkey keyforjohn.key -in message.enc -out message.txt and press Enter to decrypt the message from John.
    
27. At the cybrary@localhost prompt, type cat message.txt to and press Enter to read John's message.
    
    Success! You have now encrypted and decrypted a simple message using symmetric cryptography.